An electrochromic device which is applicable to a variety of light-modulating devices or display devices is composed of, for example, a transparent substrate, a transparent electrically conductive film, an electrochromic film colored by oxidation or reduction, an electrolyte, an electrochromic film colored by reduction or oxidation, and a transparent electrically conductive film, provided one on another in this order.
For such an electrochromic device, there is proposed to use a WO.sub.3 film as the electrochromic film colored by reduction for its practical durability and coloring efficiency. On the other hand, there has been found no material for the electrochromic film colored by oxidation, or an ion storage film which is comparable to WO.sub.3 in practical durability and coloring efficiency, and merely NiO or the like materials has been proposed.
Even when WO.sub.3 is used as the material colored by reduction in producing an electrochromic device, the performance of the electrochromic device greatly depends not only on the WO.sub.3 element, but also on the material of the counter electrode.
There have been proposed, for example, light-modulating devices for use in a sunshine roof of a vehicle having an electrode provided with an electrochromic film colored by oxidation, such as of IrO or Prussian blue opposite to the WO.sub.3 electrode. However, none of them has sufficient heat resistance, moisture resistance, UV resistance, and shake resistance for commercialization. In order to achieve the stabilized operation of a light-modulating glass under more than a hundred thousand times of electrochromic operation, the opposing electrode is required to have sufficient electrochemical stability.
For the purpose of improving the coloring efficiency, which is one of the most important factors of the electrochromic properties, Japanese Laid-open Patent Application Nos. 6-281970 and 7-134318 propose an electrode configured in the form of stripes or dots which can be used as an electrode opposite to the electrode provided with a film of a material colored by either oxidation or reduction.
In order to further improve the durability of an electrochromic device, it is essential to maintain not only the excellent electrochemical properties but also the mechanical strength of the counter electrode for a prolonged time. Such further improvement is also demanded for the counterelectrode mentioned above.